New York Mayor Michael Bloomberg is using his wealth and political acumen to back candidates who favor gun control, in a bid to counterbalance the political influence of the National Rifle Association.

New York — The National Rifle Association boasts big numbers. It has 4.5 million members and a budget of more than $300 million. It's 142 years old and is arguably unmatched in funding, focus, and fervor.

But now, New York Mayor Michael Bloomberg, along with his "super political-action committee," is emerging as a main counterweight to the NRA.

The billionaire businessman-turned-politician is pursuing an ambitious agenda to change the national conversation on guns. He brings to the table a net worth of $27 billion, a record of pursuing unpopular causes he deems important, and the defiant zeal of a man nearing the end of his career who's decided to make gun control part of his legacy.

It's clear Mr. Bloomberg won't travel an easy road. "Any longtime observer of American politics is going to put his money on the NRA," says Larry Sabato, director of the Center for Politics at the University of Virginia in Charlottesville.

But then again, Mr. Sabato says, don't count the former media mogul out. "He has enough money to be a real counterweight," he says. "Bloomberg combines almost unlimited cash with a successful politician's understanding of voters and issues, so there's no question that he tops any other NRA opponent, even longtime gun-control advocates like [Sen.] Dianne Feinstein."

Even NRA spokesman Andrew Arulanandam says: "We have billions of reasons to take him seriously," adding, "He is a well-financed opponent."

Bloomberg so far has poured roughly $12 million into his five-month-old super PAC, Independence USA, to back eight candidates in races across America. He chose them because of several issues, including marriage equality and education reform, but the top issue appears to be gun control. In particular, one race in Illinois – a special primary election in February to fill a congressional seat vacated by Jesse Jackson Jr. – put Bloomberg and his super PAC in the headlines.

One of the Democratic contenders was Debbie Halvorson, who had previously been given an 'A' rating by the NRA. The other main contender, Robin Kelly, was backed by Bloomberg. A $2.5 million flurry of spending by his super PAC, bankrolling an aggressive ad campaign, turned Ms. Halvorson's NRA rating into a scarlet letter 'A' of a liability. Ms. Kelly emerged victorious in the primary.

"The NRA for too long has held the megaphone," says John Feinblatt, chief adviser to Bloomberg. "The NRA has had the ear of Congress for too long, and the mayor believes it's important for the facts to come out."

What's on a billionaire's wish list? For Bloomberg, it's legislation requiring background checks for all gun sales, restrictions on high-capacity magazines, a ban on military-style assault weapons, and legislation making gun trafficking a felony.

He has also urged President Obama to prosecute those who lie on background checks, remove restrictions on gun-violence research, order federal agencies to provide information for a national background-check database, and appoint a director for the Bureau of Alcohol, Tobacco, Firearms, and Explosives, which has been without a leader since 2006.

"It's just an outrage, and the public, I think, should stand up," Bloomberg said in a Feb. 19 briefing. "I'm part of the public, and I happen to have some money. That's what I am trying to do with my money – trying to get us some sensible gun laws."

The mayor is taking a multipronged approach to his national agenda: money, media, and old-fashioned politicking.

Besides Kelly from Illinois, Independence USA has thrown $1 million behind then-Rep. Bob Dold (R) of Illinois, who has backed gun-control measures; $2.4 million behind Florida Democratic congressional candidate Val Demings, a member of Mayors Against Illegal Guns and a proponent of "responsible gun ownership"; and $1.1 million behind Connecticut Republican congressional candidate Andrew Roraback, who has said his party must be open to "sensible gun control."

Those three candidates, however, did not win in the November elections. (But Independence USA has tallied wins in the five other races it's entered, most of which centered on other issues.)

Independence USA's seemingly scattershot support for candidates across the geographical and political map is "all issue-based," says Stefan Friedman, a spokesman for the super PAC. "We are looking at candidates and where they stand on issues," he says.

When Independence USA supports a candidate, it usually outspends the opposition by as much as 30 to 1, leading some to accuse the mayor of buying elections. Mr. Friedman dismisses that argument as "specious."

The mayor has also been on a media circuit, promoting gun control through appearances with TV personalities including NBC's David Gregory, the women of ABC's "The View," and even Fox's Bill O'Reilly.

On Monday, Mayors Against Illegal Guns, a gun control group cofounded by Bloomberg, launched a $12 million ad campaign urging US senators in key states to support federal legislation requiring background checks for all gun purchases. The ads, scheduled to run during the Easter/Passover weekend, target 15 "persuadable" senators in 13 states: Republicans Jeff Flake of Arizona, Charles Grassley of Iowa, Susan Collins of Maine, and Rob Portman of Ohio, as well as Democrats Joe Donnelly of Indiana, Mary Landrieu of Louisiana, and Kay Hagan of North Carolina. According to The New York Times, Bloomberg is financing the campaign himself.

On March 21, he met with Vice President Joe Biden, who's led a task force on curbing gun violence, as well as families from Newtown, Conn., to discuss federal gun laws. Bloomberg has also held a closed-door meeting with former Arizona Rep. Gabrielle Giffords, herself the survivor of an assassination attempt, who has called for stricter gun laws following the Newtown shooting and has publicly chided the president for not taking leadership on gun control.

Conspicuous in many of these appearances and meetings is disdain for the NRA, which the mayor has said is engaged in "a shameful evasion of the crisis facing our country."

"The NRA is interested in doing substantive things to make sure that we reduce the problem of violence in this country," he says. "The mayor has dedicated his bank account to eradicating a constitutional right. We have determined to make sure that he fails."

The NRA does not support universal background checks or an assault weapons ban but does support restricting firearm access to "criminals and the mentally ill with violent tendencies," Arulanandam says.

"Gun bans and other bans, we don't think it's a proper or constructive way of addressing the problem of violence in our society," he says.

Mr. Feinblatt counters that it was gun violence that spurred Bloomberg to push for more gun control in the first place.

"Eighty-five percent of guns recovered from crime scenes in New York come from out of state," he says. "We have an obligation to point out that this is a national problem; it requires a national solution."

Though both sides are on a spending spree to push their respective agendas, the sums remain asymmetrical.

The NRA had a budget of $307 million in 2010. In 2011 – before Bloomberg entered the fray – "the combined budgets of all the groups trying to prevent gun violence came to around $16 million," The New York Times reported.

In last November's elections, Bloomberg's super PAC donated roughly $9.6 million to seven candidates in state races, according to Friedman. The NRA, by contrast, spent more than $25 million in the 2012 elections, most of which went to attack Mr. Obama and support GOP presidential nominee Mitt Romney, according to the Center for Responsive Politics.

To overcome the NRA, Independence USA and other gun-control groups will have to form a more well-organized, well-funded coalition, gun-debate watchers say.

"The NRA's power has been affected permanently," says John McGlennon, a political scientist at The College of William & Mary in Williamsburg, Va. "But the real key will be whether there emerges any kind of organized, permanent apparatus to carry on [Bloomberg's] campaign."

Of course, those who know the mayor, and his propensity to gravitate toward seemingly unconquerable challenges, expect him to keep up the fight long after he leaves office at the end of 2013.

"Mayor Bloomberg has had issues he's been very strong on throughout his mayoralty that he's made clear he'll continue to advocate on behalf of post-mayoralty," Friedman says. "Gun safety is one of them."